Fugitive emissions result from releases of airborne matter to the atmosphere from diffuse sources, which can include landfills, reservoirs, effluent ponds, mines, natural deposits, or even a collection of point-sources such as cities, industrial plants, or a herd of animals. Fugitive emissions can also include emissions from point sources, such as smokestacks, flares, wells, exhaust tubes, leaks and vent pipes, that have been released to the atmosphere. The airborne matters can be greenhouse gases, gaseous organic compounds, polluting gases, or particulate matter. The atmospheric volume within which the airborne matters exist is referred to as a plume. The flux is the mass flow rate per unit area. The mass flow rate is the flow rate of the airborne matter through an imaginary surface, for example downwind of an emission source, in mass per unit time. The emission discharge rate is the mass flow rate discharged by an emission source to the atmosphere in mass per unit time. The mass flow rate, if measured downwind of an emission source, and the emission discharge rate, are the same if the background concentration of airborne matter is zero and attenuation is accounted for.
Denmead (2008, Plant Soil 309:5-24) describes various approaches to measuring fluxes of methane and other subject gases between landscapes and the atmosphere. In particular, it is disclosed that mass balance methods are useful for defined source areas in the tens to thousands of square meters. Conventional micrometeorological techniques may also be employed for source areas of a similar size, but may necessitate sampling periods in the order of several minutes to hours under some conditions, or for some subject gases.
When assessing fugitive emissions from large sources such as a large landfill using current techniques, the emissions are monitored at or near the surface level, or at the edges of the landfill. Such an arrangement may involve the placement (permanently or temporarily) of a plurality of sensors or sampling devices; this placement may be limited by access to the site. Spot surface measurements may under or over estimate the emission by not detecting points with significant emissions, or by detecting a localized region of high concentration (e.g. where the gas is trapped and concentrated in a pocket or depression). Estimates and extrapolations, while useful for monitoring and modeling emission plume movement, may not be suitable in some situations where defined values are desired. Another method is to use a tracer gas and measure sample concentrations downwind of the emission source. However, tracer techniques cannot be used to determine the variation of airborne matter concentration near a diffuse source, and the tracer release pattern should mimic the emission flux pattern from the emission source.
U.S. Pat. No. 6,542,242 discloses a method for mapping of airborne matter using path-integrated optical remote sensing (ORS) with a non-overlapping variable path beam length geometry (Radial Plume Mapping). Radial Plume Mapping uses optical remote sensing instruments to obtain path-integrated data, that is processed reiteratively using a cumulative distribution function to provide a map of the concentration of airborne matters. The assumed radial concentration pattern is determined based on an assumed cumulative density function. The method, in a vertical configuration, requires a ground-based, stable vertical structure on which to mount reflectors.
U.S. Pat. No. 4,135,092 and U.S. Pat. No. 4,204,121 teach mass balance methods using either a number of totalizing samplers mounted on a vertical pole or line, an aircraft flying through the plume at various elevations collecting total samples at several height intervals, or vertically spaced infra-red radiation transmitters on a mast opposite another mast with a matching series of infra-red receptors. Sampling can be made upwind of the source area to evaluate the contribution of incoming pollution to the apparent fugitive emission rate. However, it does not teach how to determine the concentration distribution of airborne matter within the plume or account for a natural background concentration of a pollutant in the atmosphere.
Canadian patent application 2,655,279 provides a method for measurement of fugitive emission mass flow rate using an optical remote sensing instrument mounted either on an airborne platform (for ground-based targets), or with the instrument mounted on the ground and the targets mounted on an airborne platform.
Milly (1964, Int. J. Air Wat. Poll 8:291-295) describes a method for mapping of contaminant concentrations in the air using ground-based fixed masts, and samplers at various heights on each mast. The airspace to be mapped is limited by the height of the mast and the area covered by the samplers, and may not be practical for large emission plumes spanning several hectares and/or of significant height.
U.S. Pat. No. 6,750,467 and Canadian patent 2,219,335 describe a vehicle-mounted apparatus (a “GasFinder”, Boreal Laser Inc.) which allows for rapid point measurements of airborne matter concentrations. Thornton and Bowmar (A&WM Association Conference, Raleigh N.C. Oct. 28, 1999) also describe the use of the “GasFinder” (Boreal Laser Inc.).
U.S. Pat. No. 6,864,983 teaches the use of a spectrometer for receiving absorption spectra from the sun, from which emission flux can be calculated. The method depends on the availability of direct sunlight and may only be used on sunny days. In addition, the accuracy of the method for some gases is questionable due to the long absorption distance through the atmosphere. For example, the significant background concentration of methane in the atmosphere results in a very large integrated concentration of methane, compared with the contribution of most methane emission plumes.
Mapping of airborne matters can also be carried out using Differential Absorption Laser Detection and Ranging (DIAL). It can be classified as a mass balance method that uses two Nd:YAG (neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12) lasers. This equipment can map the concentration of airborne matters in the air, from which an emission flux can be calculated (Chambers et al., 15th International Emission Inventory Conference, New Orleans, La., May 2006). In an emission flux measurement application, this equipment is ground based, expensive, heavy and bulky.
U.S. Pat. No. 6,882,742 and U.S. Pat. No. 6,995,846 provide an airborne DIAL, using ND:YLF (neodymium-doped yttrium lithium fluoride; Nd:YLiF4) lasers for detection of natural gas pipeline leaks, providing a path-integrated concentration of methane and ethane. The DIAL instrument described does not map the concentration of airborne matter in the air, and there is no teaching of measuring or quantifying emission flux of the gas leak.
A method to obtain the concentration distribution of airborne matter is needed, where such measurements are obtained without the need for a tracer gas or dispersion modeling, and within a reasonable time frame across the width, depth and length of an emission plume of large area and/or height that may exceed that of ground-based moveable platforms, to provide a map of concentration of the airborne matter through a cross-section or profile of the plume. This map can then be applied to a wind velocity distribution map to obtain the emission discharge rate of airborne matter released by an emission source.
The present invention provides for a method of mapping airborne concentrations of airborne matter in an emission plume using rapid point sampling.